1. Field of the Invention
The present invention relates to an image reading apparatus, such as a copying machine, a scanner, and a facsimile, which can read surface and backside images of a document while conveying the document.
2. Description of the Related Art
Conventionally, in the image reading apparatus such as the copying machine, the scanner, and the facsimile, generally reflected light obtained in scanning a document surface with a scanning optical system including an exposure lamp and a folding mirror is imaged on a photoelectric conversion device through a lens, and the light is converted into an electric signal. Examples of the well-known document reading method include fixed scanning method of reading the document remaining stationary on a document base plate glass and a flow scanning method of reading the document which is being conveyed by an automatic document feeder (hereinafter referred to as ADF).
With reference to a method in which the document having images on both the surface and the backside is read by the flow scanning using ADF, there is a method in which one surface is read during a first conveyance using one reading unit and then the document is inverted through an inverting path provided in ADF to read the other surface. There is also a method of reading the document front side and the document backside during a one-time conveyance operation using two reading units for document front side and document backside. Recently, with a demand for enhancing a reading operation speed, frequently the two reading units are provided in the image reading apparatus. For example, the following techniques are disclosed for the image reading apparatus in which the two reading units are provided.
In a configuration disclosed in Japanese Patent Application Laid-Open No. 2000-201251, a reading unit having a reduction optical system in which an imaging lens is used is provided in a reader unit, and a contact image sensor (hereinafter referred to as CIS) in which an equal magnification optical system is used is provided in ADF. After the surface of the document conveyed by ADF is read by the reader unit, the backside is read by CIS provided in ADF.
In a configuration disclosed in Japanese Patent Application Laid-Open No. 5-37727, the reading unit having the reduction optical system in which the imaging lens is used is provided in the reader unit, and the reading unit having the reduction optical system in which the imaging lens is used is also provided in ADF. One surface of the document conveyed by ADF is read by the reading unit provided in reader unit, and the other surface is read by the reading unit provided in ADF.
In Japanese Patent Application Laid-Open No. 2002-335375, not the lens but an off-axial imaging optical system is used as the imaging optical system in the reading unit. The off-axial imaging optical system is an imaging optical system in which an imaging optical element including plural off-axial reflecting surfaces having curvatures is used. In the imaging optical system, an incident direction and an outgoing direction of a reference axis light beam are different from each other. Japanese Patent Application Laid-Open No. 2002-335375 also discloses a technique of using the reading unit in which the off-axial imaging unit having the off-axial imaging optical system is adopted.
However, in the configuration disclosed in Japanese Patent Application Laid-Open No. 2000-201251 in which the document front side is read by the reading unit having the reduction optical system while the document backside is read by CIS having the equal magnification optical system, a mismatch of color or resolution is possibly generated between reading images of the document front side and document backside due to a difference in optical system. That is, a difference in reading image between the document front side and the document backside is increased. Because different electric processes are performed to the surface and backside of the read document, two image processing circuits is required, which results in increased cost or upsizing of the apparatus.
On the other hand, in the configuration disclosed in Japanese Patent Application Laid-Open No. 5-37727 in which the reading unit having the reduction optical system is also provided in ADF to read the document backside, the problem described in Japanese Patent Application Laid-Open No. 2000-201251 is eliminated. However, in the reading unit having the reduction optical system disclosed in Japanese Patent Application Laid-Open No. 5-37727, because image deformation becomes prominent in off-axis when an angle of view of a lens is spread, it is necessary to lengthen an optical path, which results in a heavy and enlarged reading unit. Therefore, the whole apparatus also becomes enlarged and heavy, and it is not preferable that the reading unit having the reduction optical system in which the imaging lens is used be provided in ADF which is opened and closed with respect to the reader unit.
On the other hand, in the off-axial imaging optical system, because the generation of the image deformation in the off axis can be suppressed even if the optical path length is shortened, the downsizing and weight reduction can be achieved in the reading unit. Therefore, not only the off-axial imaging optical system is suitable to the reader unit, but also the off-axial imaging optical system is preferably provided in ADF which is opened and closed with respect to the reader unit. In order to decrease the difference in reading image between the document front side and document backside, preferably the off-axial imaging optical system is used in not only the reading unit for surface having the imaging optical system but also the reading unit for backside having the imaging optical system. For example, the reading unit in which the off-axial imaging unit is used is provided in ADF while the reading unit in which the off-axial imaging unit is used is provided in the reader unit, and thereby reading the document front side and document backside.
However, in this case, the off-axial imaging unit provided in ADF becomes upside down with respect to the off-axial imaging unit provided in the reader unit. That is, an incident side of a reference axis light beam is located below an outgoing side in the off-axial imaging unit provided in the reader unit, while the incident side of the light beam is located above the outgoing side in the off-axial imaging unit provided in ADF. In the off-axial imaging unit, because the imaging optical element including an off-axial reflecting surface is generally made of a resin material, the imaging optical element has low strength against an external force. When the imaging optical element becomes upside down to change a direction of gravity acting on the imaging optical element, the off-axial reflecting surface is deformed to change optical performance. Therefore, performances cannot be kept constant in the reading unit provided in ADF and the reading unit provided in the reader unit. This causes the difference in reading image between the document front side and the document backside to be increased.
The cost is increased when the strength of the off-axial imaging unit provided in ADF is increased with respect to the off-axial imaging unit provided in the reader unit in order to deal with the deformation of the off-axial reflecting surface due to the gravity.
In view of the foregoing, the invention is to prevent the change in imaging performance caused by the deformation of the off-axial reflecting surface due to the gravity while preventing the difference in reading image between the document front side and the document backside, when the off-axial imaging unit is used in the reading devices which read the document front side and the document backside respectively. The invention also prevents the deformation of the off-axial reflecting surface due to the gravity without increasing the coast.